Head and Neck

Cranial Fossae

The cranial base is the floor of the neurocranium, which supports the brain, and is divided into the following three cranial fossae ( Fig. 8.4 ):

• Anterior: the roof of the orbits and the midline nasal cavity; accommodates the frontal lobes of the brain.

• Middle: accommodates the temporal lobes of the brain.

• Posterior: accommodates the cerebellum, pons, and medulla oblongata of the brain.

Each fossa has numerous foramina (openings) for structures to pass in or out of the neurocranium.

Meninges

The brain and spinal cord are surrounded by three membranous connective tissue layers called the meninges, which include the following ( Figs. 1.21 and 8.5 ):

• Dura mater: thick outermost meningeal layer that is richly innervated by sensory nerve fibers.

• Arachnoid mater: fine, weblike avascular membrane directly beneath the dural surface; the space between the arachnoid mater and the underlying pia mater is called the subarachnoid space and contains cerebrospinal fluid, which bathes and protects the central nervous system (CNS).

• Pia mater: delicate membrane of connective tissue that intimately envelops the brain and spinal cord.

The cranial dura mater is distinguished from the dura mater covering the spinal cord by its two layers. An outer periosteal layer is attached to the inner aspect of the cranium and is supplied by the meningeal arteries, which lie on its surface between it and the bony skull. Imprints of these meningeal artery branches can be seen as depressions on the inner table of bone. This periosteal dura mater is continuous with the periosteum on the outer surface of the skull at the foramen magnum and where other intracranial foramina open onto the outer skull surface. The inner dural layer is termed the meningeal layer and is in close contact with the underlying arachnoid mater and is continuous with the spinal dura mater at the level of the foramen magnum.

The dura mater is richly innervated by meningeal sensory branches of the trigeminal nerve (fifth cranial nerve, CN V); the vagus nerve (CN X), specifically to the posterior cranial fossa; and the upper cervical nerves . A portion of the dura mater in the posterior cranial fossa also may receive some innervation from the glossopharyngeal nerve (CN IX), accessory nerve (CN XI), and hypoglossal nerve (CN XII). The arachnoid mater and pia mater lack sensory innervation. The periosteal dura mater and meningeal dura mater separate to form thick connective tissue folds or layers that separate various brain regions and lobes ( Figs. 8.5–8.8 ):

• Falx cerebri: double layer of meningeal dura mater between the two cerebral hemispheres.

• Falx cerebelli: sickle-shaped layer of meningeal dura mater that projects between the two cerebellar hemispheres.

• Tentorium cerebelli: fold of meningeal dura mater that covers the cerebellum and supports the occipital lobes of the cerebral hemispheres.

• Diaphragma sellae: horizontal shelf of meningeal dura mater that forms the roof of the sella turcica covering the pituitary gland; the infundibulum passes through this dural shelf to connect the hypothalamus with the pituitary gland.

Dural Venous Sinuses

The dura mater also separates to form several large endothelial-lined venous channels between its periosteal and meningeal layers; these include the superior and inferior sagittal sinuses, straight sinus, confluence of sinuses, transverse, sigmoid, and cavernous sinuses, and several smaller dural sinuses ( Table 8.2 and Fig. 8.7 ). These dural venous sinuses drain blood from the brain, largely posteriorly, and then largely into the internal jugular veins. These sinuses lack valves, however, so the direction of blood flow through the sinuses is pressure dependent. Of particular importance is the cavernous venous sinus ( Fig. 8.7 ), which lies on either side of the sella turcica and has an anatomical relationship with the internal carotid artery and several cranial nerves, including CN III, CN IV, CN V ₁ , CN V ₂ , and CN VI. Injury or inflammation in this region can affect some or all of these important structures. Also, the optic chiasm lies just above this area, so CN II may be involved in any superior expansion of the cavernous sinus (e.g., pituitary tumor).

Subarachnoid Space

The subarachnoid space (between the arachnoid mater and pia mater) contains cerebrospinal fluid (CSF), which performs the following functions ( Figs. 8.5 and 8.8 ):

• Supports and cushions the spinal cord and brain.

• Fulfills some functions normally provided by the lymphatic system by draining some CSF macromolecules into the meningeal dural lymphatics (the CNS glymphatic system).

• Occupies a volume of about 150 mL in the subarachnoid space.

• Is produced by choroid plexuses in the brain’s ventricles.

• Is produced at a rate of about 500 to 700 mL/day.

• Is reabsorbed largely by the cranial arachnoid granulations and by microscopic arachnoid granulations feeding into venules along the length of the spinal cord.

The arachnoid granulations absorb most of the CSF and deliver it to the dural venous sinuses (see Figs. 8.5 and 8.8 ). These granulations are composed of convoluted aggregations of arachnoid mater that extend as “tufts” into the superior sagittal sinus and function as one-way valves for the clearance of CSF; the CSF crosses into the venous sinus, but venous blood cannot enter the subarachnoid space. Small, microscopic arachnoid cell herniations also occur along the spinal cord, where CSF (which circulates at a higher pressure than venous blood) is delivered directly into small spinal cord veins. The CSF circulating around the brain (and spinal cord) provides a protective cushion and buoyancy for the CNS, thus reducing the pressure of the brain on the vessels and nerves on its inferior surface. CSF also can serve as a fluid delivery system for certain chemical mediators (e.g., interleukins and prostaglandins), clears the CNS of metabolites directly or via the glymphatic system, and represents an internal paracrine communication system for certain CNS areas that are close to the ventricles.

Clinical Focus 8.4

Hydrocephalus

Hydrocephalus is the accumulation of excess CSF within the brain’s ventricular system. It is caused by overproduction or decreased absorption of CSF or by blockage of one of the passageways for CSF flow in the subarachnoid space.

Clinical Focus 8.5

Meningitis

Meningitis is a serious condition defined as an inflammation of the arachnoid mater and pia mater. It results most often from bacterial or aseptic causes. Aseptic causes include viral infections, drug reactions, and systemic diseases. Patients with meningitis usually present with the following symptoms:

• Headache

• Fever

• Seizures

• Painful stiff neck

Diagnosis is made by performing a lumbar puncture and examining the CSF.

Gross Anatomy of the Brain

The most notable feature of the human brain is its large cerebral hemispheres ( Figs. 8.9 and 8.10 ). Several circumscribed regions of the cerebral cortex are associated with specific functions, and key surface landmarks of the typical human cerebrum are used to divide the brain into lobes: four or five, depending on classification, with the fifth lobe being either the insula or the limbic lobe. The lobes and their general functions are as follows:

• Frontal: mediates precise voluntary motor control, learned motor skills, planned movement, eye movement, expressive speech, personality, working memory, complex problem solving, emotions, judgment, socialization, olfaction, and drive.

• Parietal: affects sensory input, spatial discrimination, sensory representation and integration, taste, and receptive speech.

• Occipital: affects visual input and processing.

• Temporal: mediates auditory input and auditory memory integration, spoken language (dominant side), and body language (nondominant side).

• Insula: a fifth deep lobe that lies medial to the temporal lobe (sometimes included as part of temporal lobe) ; influences vestibular function, some language, perception of visceral sensations (e.g., upset stomach), emotions, and limbic functions.

• Limbic: also sometimes considered a fifth medial lobe (cingulate cortex); influences emotions and some autonomic functions.

Other key areas of the brain include the following components ( Fig. 8.9 ):

• Thalamus: gateway to the cortex; simplistically functions as an “executive secretary” to the cortex (relay center between cortical and subcortical areas).

• Cerebellum: coordinates smooth motor activities, and processes muscle position; possible role in behavior and cognition.

• Brainstem: includes the midbrain, pons, and medulla oblongata; conveys motor and sensory information from the body and autonomic and motor information from higher centers to peripheral targets.

Internally, the brain contains four ventricles, two lateral ventricles, and a central third and fourth ventricle ( Fig. 8.11 ). Cerebrospinal fluid , produced by the choroid plexus (see Fig. 8.5 ), circulates through these ventricles and then enters the subarachnoid space through two lateral apertures (foramina of Luschka) or a median aperture (foramen of Magendie) in the fourth ventricle ( Fig. 8.11 ).

Blood Supply to the Brain

Arteries supplying the brain arise largely from the following two pairs of arteries ( Fig. 8.12 and Table 8.3 ):

• Vertebrals: these two arteries (right and left) arise from the subclavian artery, ascend through the transverse foramina of the C1-C6 vertebrae, and enter the foramen magnum of the skull.

• Internal carotids: these two arteries (right and left) arise from the common carotid artery in the lower neck, ascend superiorly in the neck, enter the carotid canal, and traverse the foramen lacerum to terminate as the middle and anterior cerebral arteries, which anastomose with the arterial circle of Willis.

Clinical Focus 8.6

Subarachnoid Hemorrhage

Subarachnoid hemorrhage usually occurs from an arterial source and results in the collection of blood between the arachnoid mater and pia mater . The most common cause of subarachnoid hemorrhage is the rupture of a saccular, or berry, aneurysm .

The vertebral arteries give rise to the anterior and posterior spinal arteries (a portion of the supply to the spinal cord; see Fig. 2.20 ) and the posterior inferior cerebellar arteries, and then join at about the level of the junction between the medulla and pons to form the basilar artery ( Fig. 8.12 ). The internal carotid arteries each give rise to an ophthalmic artery, a posterior communicating artery, a middle cerebral artery, and an anterior cerebral artery. Table 8.3 summarizes the brain regions supplied by these vessels and their major branches.

Clinical Focus 8.7

Epidural Hematomas

Epidural hematomas result most often from motor vehicle crashes, falls, and sports injuries. The blood collects between the periosteal dura mater and bony cranium . The source of the bleeding is usually arterial (85%); common locations include the frontal, temporal (middle meningeal artery is very susceptible, especially where it lies deep to the pterion), and occipital regions.

Clinical Focus 8.8

Subdural Hematomas

Subdural hematomas are usually caused by an acute venous hemorrhage of the cortical bridging veins draining cortical blood into the superior sagittal sinus. Half are associated with skull fractures. In a subdural hematoma the blood collects between the meningeal dura mater and the arachnoid mater (a potential space). Clinical signs include a decreasing level of consciousness, ipsilateral pupillary dilation, headache, and contralateral hemiparesis. These hematomas may develop within 1 week after injury but often present with clinical signs within hours. Chronic subdural hematomas are most common in elderly persons and alcoholic patients who have some brain atrophy, which increases the space traversed by the bridging veins and renders the stretched vein susceptible to tearing.

Clinical Focus 8.9

Transient Ischemic Attack

A transient ischemic attack (TIA) is a temporary interruption of focal brain circulation that results in a neurologic deficit that lasts less than 24 hours, usually 15 minutes to 1 hour. The most common cause of TIA is embolic disease from the heart, carotid, or cerebral vessels, which may temporarily block a vessel. The onset of the deficit is abrupt, and recovery is gradual. The most common deficits include the following:

• Hemiparesis

• Hemisensory loss

• Aphasia

• Confusion

• Hemianopia

• Ataxia

• Vertigo

Clinical Focus 8.10

Stroke

Cerebrovascular accident (CVA) or stroke is a localized brain injury caused by a vascular episode that lasts more than 24 hours, whereas a transient ischemic attack (TIA) is a focal ischemic episode lasting less than 24 hours. Stroke is classified into the following two types:

• Ischemic (70-80%): infarction; thrombotic or embolic, resulting from atherosclerosis of the extracranial (usually carotid) and intracranial arteries or from underlying heart disease.

• Hemorrhagic: occurs when a cerebral vessel weakens and ruptures (subarachnoid or intracerebral hemorrhage), which causes intracranial bleeding, usually affecting a larger brain area.

Clinical Focus 8.11

Carotid–Cavernous Sinus Fistula

More common than symptomatic intracavernous sinus aneurysms but less common than subarachnoid saccular (berry) aneurysms, carotid–cavernous sinus fistulas often result from trauma and are more common in men. These high-pressure (arterial) low-flow lesions are characterized by an orbital bruit, exophthalmos, chemosis, and extraocular muscle palsy involving CN III, IV, and VI . Blood collecting in the cavernous sinus drains by several venous pathways because the sinus has connections with other dural venous sinuses as well as with the ophthalmic veins and pterygoid plexus of veins in the infratemporal region.

Clinical Focus 8.12

Collateral Circulation After Internal Carotid Artery Occlusion

If a major artery such as the internal carotid becomes occluded, extracranial and intracranial (circle of Willis) anastomoses may provide collateral routes of circulation. These routes are more likely to develop when occlusion is gradual, as in atherosclerosis, rather than acute, as in embolic obstruction.

Clinical Focus 8.13

Vascular (Multiinfarct) Dementia

Dementia is an acquired neurologic syndrome that presents with multiple cognitive deficits. By definition, dementia includes short-term memory impairment, behavioral disturbance, and/or difficulties with daily functioning and independence. Dementia can be classified as degenerative, vascular, alcoholic, or human immunodeficiency virus (HIV) related. Vascular dementias are caused by anoxic damage from small infarcts and account for about 15% to 20% of dementia cases. Multiinfarct dementia is associated with heart disease, diabetes mellitus, hypertension, and inflammatory diseases.

Clinical Focus 8.14

Brain Tumors

Clinical signs and symptoms of brain tumors depend on the location and the degree to which intracranial pressure (ICP) is elevated. Slow-growing tumors in relatively silent areas (e.g., frontal lobes) may go undetected and can become quite large before symptoms occur. Small tumors in key brain areas can lead to seizures, hemiparesis, or aphasia. Increased ICP can initiate broader damage by compressing critical brain structures. Early symptoms of increased ICP include malaise, headache, nausea, papilledema, and less often abducens nerve palsy and Parinaud’s syndrome. Classic signs of hydrocephalus are loss of upward gaze, downward ocular deviation (“setting sun” syndrome), lid retraction, and light-near dissociation of pupils. Primary tumors include the following :

• Gliomas: arise from astrocytes or oligodendrocytes; glioblastoma multiforme is the most malignant form (astrocytic series).

• Meningiomas: arise from the arachnoid mater and can extend into the brain.

• Pituitary tumors: can expand in the sella turcica and affect CN II, III, IV, V ₁ , V ₂ , and VI; about 15% of primary tumors.

• Neuromas: acoustic neuroma, a benign tumor of CN VIII, is a common example; about 7% of primary tumors.

Clinical Focus 8.15

Metastatic Brain Tumors

Metastatic brain tumors are more common than primary brain tumors. Most spread via the bloodstream, with cells seeded between the white matter (fiber tract pathways) and gray matter (cortical neurons). Some tumors metastasize directly from head and neck cancers or through Batson’s vertebral venous plexus. Presentation often includes headache (50%), seizures (25%), and elevated intracranial pressure.

Cranial Nerves

See Chapter 1 for an overview of the general organization of the nervous system.

In addition to the 31 pairs of spinal nerves, 12 pairs of cranial nerves arise from the brain and upper spinal cord (CN XI). As with the spinal nerves, cranial nerves are part of the periph­eral nervous system and are identified both by name and by Roman numerals CN I to CN XII ( Fig. 8.13 ). Cranial nerves are somewhat unique and may contain the following multiple functional components:

• General (G): same general functions as spinal nerves.

• Special (S): functions found only in cranial nerves (special senses of vision, hearing, and balance, and the sensations of smell and taste).

• Afferent (A) or efferent (E): sensory or motor functions, respectively.

• Somatic (S) or visceral (V): related to skin and skeletal muscle innervation ( somatic ), or to smooth muscle, cardiac muscle, and glands ( visceral ).

By convention, each cranial nerve is classified as either general (G) or special (S), and then somatic (S) or visceral (V), and finally as afferent (A) or efferent (E). For example, a cranial nerve that is classified GVE (general visceral efferent) means it contains motor fibers to visceral structures, such as parasympathetic fibers in the vagus nerve.

In general, cranial nerves are described as follows ( Table 8.4 ):

• CN I and II: arise from the forebrain; are really tracts of the brain for the special senses of smell and sight, respectively; they are brain extensions surrounded by all three meningeal coverings, with CSF in the subarachnoid space—but still are classified as cranial nerves.

• CN III, IV, and VI: move the extraocular skeletal muscles of the eyeball.

• CN V: has three divisions; V ₁ and V ₂ are sensory, and V ₃ is both sensory and motor.

• CN VII, IX, and X: are both motor and sensory.

• CN VIII: is the special sense of hearing and balance, but unlike CN I and II, is not a brain tract.

• CN XI and XII: are motor to skeletal muscle.

• CN III, VII, IX, and X: also contain parasympathetic (visceral) fibers of origin, although many of these autonomic fibers “jump” onto branches of CN V to reach their targets, because the branches of CN V pass almost everywhere in the head.

Rather than describe each cranial nerve and all its branches in detail at this time, we will review each nerve anatomically and clinically as we encounter it in the various regions of the head and neck. It may be helpful to refer back to this section each time you are introduced to a new region and its cranial nerve innervation. Autonomic components of the cranial nerves (parasympathetics) and their autonomic ganglia are summarized in Fig. 1.26 and Table 1.4 . All of the cranial nerves and their components also are summarized at the end of this chapter.

Layers of the Scalp

The layers of the SCALP include the following ( Fig. 8.8 ):

• S kin.

• C onnective tissue that contains a rich supply of blood vessels of the scalp; lacerations of the scalp bleed profusely because this dense connective tissue layer often holds the vessels open and prevents their retraction into the tissue.

• A poneurosis (galea aponeurotica) of the epicranial muscles (frontalis and occipitalis).

• L oose connective tissue deep to the aponeurosis, which contains emissary veins that communicate with the cranial diploë and dural sinuses within the cranium.

• P eriosteum (pericranium) on the surface of the bony skull.

The loose connective tissue layer allows the skin to move over the skull when one rubs the head and also allows infections to spread through this layer ( Fig. 8.8 ). Small emissary veins communicate with this layer and can pass infections intracranially.

Muscles of Facial Expression

The muscles of facial expression are skeletal muscles that lie in the subcutaneous tissue of the face. They are all innervated by the terminal motor branches of the facial nerve (CN VII) ,and most originate from the underlying facial skeleton but insert into the skin or facial cartilages ( Fig. 8.14 ). Table 8.5 summarizes several of the major facial muscles, which are derived from the second branchial embryonic arch (see Embryology ) and are often referred to as branchial (branchiomeric) muscles. These muscles are skeletal muscles, but their derivation from the branchial arches means they are innervated by cranial nerves rather than spinal nerves.

Innervation of the facial muscles is by the five terminal branches of CN VII. The facial nerve enters the internal acoustic meatus of the skull, passes through the facial canal in the petrous portion of the temporal bone, and then descends to emerge from the stylomastoid foramen. CN VII then passes through the parotid salivary gland, and its terminal branches are distributed over the face and neck ( Fig. 8.15 ). The five terminal motor (branchial motor) branches are as follows:

• Temporal.

• Zygomatic.

• Buccal.

• Marginal mandibular.

• Cervical.

The sensory innervation of the face is by the three divisions of the trigeminal nerve (CN V), with some contributions by the cervical plexus. Fig. 8.16 lists the specific nerves for each division. All the sensory neurons in CN V reside in the trigeminal (semilunar, gasserian) ganglion. The trigeminal nerve is divided as follows:

• Ophthalmic (CN V ₁ ) division: exits the skull via the superior orbital fissure.

• Maxillary (CN V ₂ ) division: exits the skull via the foramen rotundum.

• Mandibular (CN V ₃ ) division: exits the skull via the foramen ovale.

Clinical Focus 8.16

Trigeminal Neuralgia

Trigeminal neuralgia (or tic douloureux ) is a neurologic condition characterized by episodes of brief, intense facial pain over one of the three areas of distribution of CN V. The pain is so intense that the patient winces, which produces a facial muscle tic.

Clinical Focus 8.17

Herpes Zoster (Shingles)

Herpes zoster, or shingles, is the most common infection of the peripheral nervous system (PNS). It is an acute neuralgia confined to the dermatome distribution of a specific spinal or cranial sensory nerve root.

Clinical Focus 8.18

Facial Nerve (Bell’s) Palsy

Acute, unilateral idiopathic facial palsy is the most common cause of facial muscle weakness and cranial neuropathy. Facial nerve palsy also may be caused by herpes simplex virus (HSV) infection. Manifestations associated with lesions at various points along the path of CN VII are illustrated.

Clinical Focus 8.19

Tetanus

The PNS motor unit is vulnerable to three bacteria-produced toxins: tetanospasmin (motor neuron), diphtheria toxin (peripheral nerve), and botulin (neuromuscular junction). The hearty spore of Clostridium tetani is commonly found in soil, dust, and feces and can enter the body through wounds, blisters, burns, skin ulcers, insect bites, and surgical procedures. Symptoms include restlessness, low-grade fever, and stiffness or soreness. Eventually, nuchal rigidity, trismus (lockjaw), dysphagia, laryngospasm, and acute, massive muscle spasms can occur. Prophylaxis (immunization) is the best management.

The blood supply to and venous drainage from the face includes the following vessels ( Fig. 8.17 ):

• Facial artery: arises from the external carotid artery.

• Superficial temporal artery: one of the two terminal branches of the external carotid artery.

• Ophthalmic artery : arises from the internal carotid artery and distributes its terminal branches over the forehead.

• Facial vein: drains into the internal jugular vein, directly or as a common facial vein.

• Retromandibular vein: formed by the union of the maxillary and superficial temporal veins; ultimately drains into the external and/or the internal jugular vein.

• Ophthalmic veins: tributaries from the forehead drain into superior and inferior ophthalmic veins in the orbit (and also anastomose with the facial vein) and then posteriorly into the cavernous dural sinus and/or the pterygoid plexus of veins in the infratemporal region (see Fig. 8.32 ).

Bony Orbit

The bones contributing to the orbit include the following ( Fig. 8.18 ):

• Frontal (orbital surface).

• Maxilla (orbital surface).

• Zygomatic (orbital surface).

• Sphenoid.

• Palatine (orbital plate).

• Ethmoid (orbital plate).

• Lacrimal.

The back of the orbit has three large openings that include the following:

• Superior orbital fissure: CN III, IV, VI, and V ₁ (frontal, lacrimal, and nasociliary nerves) pass through the fissure along with the ophthalmic vein.

• Inferior orbital fissure: CN V ₂ and infraorbital vessels pass through this fissure.

• Optic canal: CN II and the ophthalmic artery pass through this canal.

The periosteum of the orbital bones is a distinct layer of connective tissue called the periorbita. It is continuous with the pericranium (periosteum) covering the skull and, where the orbit communicates with the cranial cavity (e.g., superior orbital fissure), the periorbita is continuous with the periosteal layer of the dura mater.

Eyelids and Lacrimal Apparatus

The eyelids protect the eyeballs and keep the corneas moist. Each eyelid contains a tarsal plate of dense connective tissue; tarsal glands that secrete an oily mixture into the tears; modified sebaceous glands associated with each eyelash; apocrine glands (modified sweat glands); accessory lacrimal glands along the inner surface of the upper eyelid; and in the superior eyelid only, a small slip of smooth muscle (superior tarsal [Müller’s] muscle) , which attaches to the tarsal plate along with the levator palpebrae superioris muscle ( Fig. 8.19 ). The tears contain albumins, lactoferrin, lysozyme, lipids, metabolites, and electrolytes. The lacrimal glands secrete continuously, and as one blinks, the tears are evenly spread across the conjunctiva and cornea. Tears not only keep the eye surface moist but also possess antimicrobial properties. The lacrimal apparatus includes the following structures ( Fig. 8.19 ):

• Lacrimal glands: secrete tears; innervated by the facial nerve postganglionic parasympathetic fibers.

• Lacrimal ducts: excretory ducts of the glands.

• Lacrimal canaliculi: collect tears into openings on the medial aspect of each lid called the puncta, and convey them to the lacrimal sacs.

• Lacrimal sacs: collect tears and release them into the nasolacrimal duct when one blinks (contraction of the orbicularis oculi muscle).

• Nasolacrimal ducts: convey tears from lacrimal sacs to the inferior meatus of the nasal cavity.

Clinical Focus 8.20

Orbital Blow-Out Fracture

A massive zygomaticomaxillary complex fracture or a direct blow to the front of the orbit (e.g., by baseball or fist) may cause a rapid increase in intraorbital pressure resulting in a blow-out fracture of the thin orbital floor . In severe comminuted fractures of the orbital floor, the orbital soft tissues may herniate into the underlying maxillary paranasal sinus. Clinical signs include diplopia, infraorbital nerve paresthesia, enophthalmos, edema, and ecchymosis.

The lacrimal glands receive secretomotor parasympathetic fibers from the facial nerve (CN VII) that originate in the superior salivatory nucleus. These preganglionic parasympathetic fibers travel in the greater petrosal nerve and in the nerve of the pterygoid canal (vidian nerve), and the fibers then synapse in the pterygopalatine ganglion. Postganglionic parasympathetic fibers travel through the maxillary nerve (CN V ₂ ), zygomatic nerve, and lacrimal nerve (CN V ₁ ) to the lacrimal gland (see Figs. 8.72 and 8.73 ). Some of the postganglionic sympathetic nerves from the superior cervical ganglion (SCG) pass from the internal carotid plexus and form the deep petrosal nerve, join the greater petrosal nerve, and form the nerve of the pterygoid canal . These postganglionic sympathetic fibers (largely vasomotor in function) then follow the same course up to the lacrimal glands. The sensory innervation of the lacrimal gland is through the ophthalmic division of the trigeminal nerve (via the lacrimal branch).

Muscles

The orbital muscles include six extraocular skeletal muscles that move the eyeball and one skeletal muscle that elevates the upper eyelid ( Fig. 8.20 and Table 8.6 ). In addition to the movements of elevation, depression, abduction, and adduction, the superior rectus and superior oblique muscles medially rotate (intorsion) the eyeball, and the inferior rectus and inferior oblique muscles laterally rotate (extorsion) the eyeball. The actions of the extraocular muscles detailed in Table 8.6 reflect their “anatomical” actions; because of how the muscles insert into the globe, any single action of the eye often involves multiple muscles contracting at the same time. For example, two muscles elevate the eyeball (superior rectus and inferior oblique muscles), and three muscles abduct the eyeball (lateral rectus, superior oblique, and inferior oblique muscles). Clinically, one needs to “isolate” the multiple actions of the muscles so that an individual muscle’s action can be assessed (e.g., elevation or depression; see Clinical Focus 8.21 ). Therefore, it is important for clinicians to understand how to “clinically assess” the individual extraocular muscles, because this is how one will diagnose extraocular muscle weakness and/or nerve lesions (CN III, CN IV, and CN VI) ( Clinical Focus 8.21 ).

The levator palpebrae superioris muscle elevates the upper eyelid and, from its distal inferior surface, has a small amount of smooth muscle (superior tarsal muscle) connecting it to the tarsal plate ( Fig. 8.19 , bottom image). This smooth muscle is innervated by postganglionic sympathetic fibers from the superior cervical ganglion. The interruption of this sympathetic pathway can lead to a moderate or partial ptosis, or drooping, of the upper eyelid ipsilaterally. Interruption of the innervation of the levator palpebrae superioris from CN III, on the other hand, can lead to a significant ptosis.

Clinical Focus 8.21

Clinical Testing of the Extraocular Muscles

Because extraocular muscles act as synergists and antagonists and may be responsible for multiple movements (see Table 8.6 ), it is difficult to test each muscle individually. However, the generalist physician can check extraocular muscle (or nerve) impairment by assessing the ability of individual muscles to elevate or depress the globe with the eye abducted or adducted, thereby aligning the globe with the pull (line of contraction) of the muscle. Generally, intorsion and extorsion are too difficult to assess in a routine eye examination. The examiner can use an H pattern to assess how each eye tracks movement of an object (the tester’s finger). For example, when the finger is held up and to the right of the patient’s eyes, the patient must primarily use the superior rectus (SR) muscle of the right eye and the inferior oblique (IO) muscle of the left eye to focus on the finger. Pure abduction is done by the lateral rectus muscle, and pure adduction is done by the medial rectus muscle. In all other cases, two muscles elevate the eye (SR and IO, with minimal intorsion or extorsion) and two muscles depress the eye (inferior rectus and superior oblique muscles, with minimal intorsion or extorsion) in abduction and adduction, respectively. At the end of this test, the examiner can bring the finger directly to the midline to test convergence (medial rectus muscles of both eyes). If an eye movement disorder is detected by this method, a clinical specialist may be consulted for further evaluation.

Clinical Focus 8.22

Horner’s Syndrome

Horner’s syndrome occurs when there is a lesion somewhere along the pathway of the sympathetic fibers traveling to the head, usually from the sympathetic trunk distally. The cardinal signs are as follows:

• Ptosis: partial drooping of the upper eyelid on the affected side caused by paralysis of the superior tarsal smooth muscle in the free edge of the levator palpebrae superioris muscle.

• Miosis: pupillary constriction on the affected side caused by the paralysis of the pupillary dilator smooth muscle in the iris.

• Anhidrosis: loss of sweating on the affected side of the head caused by loss of sweat gland innervation by the sympathetic fibers.

• Flushed, warm dry skin: vasodilation of the subcutaneous arteries on the affected side caused by a lack of sympathetic vasoconstriction tone and sweat gland innervation.

Nerves in the Orbit

Three cranial nerves innervate the extraocular skeletal muscles (CN III, CN IV, and CN VI) ( Table 8.6 ). Additionally, one cranial nerve mediates the special sense of sight (CN II), and one cranial nerve conveys general sensory information from the orbit and eye (CN V ₁ ) ( Fig. 8.21 ). The major branches of the ophthalmic nerve (CN V ₁ ) include the following:

• Frontal: runs on the superior aspect of the levator palpebrae superioris muscle and ends as the supratrochlear and supraorbital nerves; sensory to forehead, scalp, frontal sinus, and upper eyelid.

• Lacrimal: courses laterally on the superior aspect of the lateral rectus muscle to the lacrimal gland; sensory to conjunctiva and skin of the upper eyelid, and the lacrimal gland.

• Nasociliary: gives rise to short and long ciliary nerves, posterior and anterior ethmoidal nerves, and infratrochlear nerve; sensory to iris and cornea, sphenoid and ethmoid sinuses, lower eyelid, lacrimal sac, and skin of the anterior nose.

The optic nerve (CN II) is actually a brain tract that conveys sensory information from the retina, via the ganglion cell axons, to the brain (see Fig. 8.13 ). The optic nerve is covered by the same three meningeal layers as the rest of the CNS, and the retina is really our “window” into the brain (see Clinical Focus 8.25 ).

In addition to supplying five of the seven skeletal muscles in the orbit (see Table 8.6 ), the oculomotor nerve (CN III) also provides parasympathetic fibers, which exhibit the following features (see Figs. 8.69 and 8.71 ):

• Preganglionic parasympathetic fibers arise centrally from the nucleus of Edinger-Westphal (accessory oculomotor nucleus) and course along CN III and its inferior division to synapse in the ciliary ganglion on postganglionic parasympathetic neurons.

• Postganglionic parasympathetic fibers then course via short ciliary nerves to the eyeball.

• These postganglionic fibers innervate the sphincter muscle of the pupil (sphincter pupillae) and the ciliary muscle for accommodation of the lens.

Sympathetic innervation to the eyeball is arranged as follows (see Figs. 8.69–8.71 ):

• Preganglionic sympathetic nerve fibers arise from the upper thoracic intermediolateral cell column of the spinal cord (T1-T2) and send preganglionic fibers into the sympathetic trunk, where these fibers ascend to synapse in the superior cervical ganglion (SCG).

• Postganglionic sympathetic fibers course along the internal carotid artery, enter the orbit on the ophthalmic artery and ophthalmic nerve, and pass through the ciliary ganglion or along the long and short ciliary nerves to the eyeball.

• These postganglionic fibers innervate the dilator muscle of the pupil (dilator pupillae) and the superior tarsal muscle by joining the oculomotor nerve to the upper eyelid.

Eyeball (Globe)

The human eyeball measures about 25 mm in diameter, is tethered in the bony orbit by six extraocular muscles that move the globe, and is cushioned by fat that surrounds the posterior two-thirds of the globe ( Fig. 8.22 ). The outer fibrous white coat of the eyeball is the sclera and is continuous anteriorly with the transparent cornea. A middle vascular layer called the choroid is continuous anteriorly with the ciliary body, ciliary process, and iris . It provides oxygen and nutrients to the underlying retina. The inner layer is the optically receptive retina posteriorly and an anterior nonvisual retinal extension that lines the internal surface of the ciliary body and iris ( Table 8.7 ).

The large chamber behind the lens is the vitreous chamber (body) and is filled with a gel-like substance called the vitreous humor, which helps cushion and protect the fragile retina during rapid eye movements (see Fig. 8.22 ).

The chamber between the cornea and the iris is the anterior chamber; the space between the iris and lens is the posterior chamber. Both chambers are filled with aqueous humor, which is produced by the ciliary body and circulates from the posterior chamber, through the pupil, and into the anterior chamber, where it is absorbed by the trabecular meshwork into the scleral venous sinus (canal of Schlemm) at the angle of the cornea and iris.

Retina

The retina consists of the optic or neural retina, which is sensitive to light, and the nonvisual retina, which lines the internal surface of the ciliary body and iris. The junction separating the neural from the nonvisual retina is called the ora serrata (see Fig. 8.22 ).

The neural retina is composed of an outer retinal pigmented epithelium lying adjacent to the vascular choroid and a photosensitive region consisting of photoreceptive cells: rods are more sensitive to light and are the receptors for low-light conditions (gray tones); cones are less sensitive to low light but are very sensitive to red, green, and blue regions of the visual spectrum. Interspersed layers of conducting and association neurons and supporting cells lie more internally in the retina, closer to the vitreous body.

The axons of ganglion cells ultimately convey the photosensory information to the optic disc, where the axons course in the optic nerve and are relayed centrally. The optic disc is our “blind spot” because no cones or rods are present in this region of the retina.

Clinical Focus 8.23

Eyelid Infections and Conjunctival Disorders

Clinical Focus 8.24

Papilledema

The optic nerve is a tract of the brain and is therefore surrounded by the three meningeal layers that cover the CNS. The subarachnoid space extends along the nerve to the point where it attaches to the posterior aspect of the eyeball. If intracranial pressure is increased, this pressure also compresses the optic nerve and its venous return through the retinal veins. This results in edema of the optic disc, which can be detected by ophthalmoscopic examination (see Fig. 8.22 ).

Clinical Focus 8.25

Diabetic Retinopathy

Diabetic retinopathy develops in almost all patients with type 1 diabetes mellitus (DM) and in 50% to 80% of patients with type 2 DM of 20 years’ duration or more. Retinopathy can progress rapidly in pregnant women with type 1 DM. Diabetic retinopathy is the number-one cause of blindness in middle-aged individuals and the fourth leading cause of blindness overall in the United States.

Clinical Focus 8.26

Glaucoma

Glaucoma is an optic neuropathy that can lead to visual field deficits and is often associated with elevated intraocular pressure (IOP).

Clinical Focus 8.27

Ocular Refractive Disorders

Ametropia is the aberrant focusing of light rays on a site other than the optimal site on the retina (macula). Optically, the cornea, lens, and axial length of the eyeball must be in precise balance to achieve sharp focus on the macula. Common disorders include the following:

• Myopia: nearsightedness; 80% of ametropias (see close objects better)

• Hyperopia: farsightedness; age-related occurrence (see distant objects better)

• Astigmatism: nonspherical cornea causes focusing at multiple locations instead of at a single point; affects 25% to 40% of the U.S. population.

• Presbyopia: age-related progressive loss of accommodative ability (lens is less flexible); see distant objects better than close objects, e.g., trouble reading small type.

Clinical Focus 8.28

Cataract

A cataract is an opacity, or cloudy area, in the crystalline lens. Risk factors for cataracts include age, smoking, alcohol use, sun exposure, low educational status, diabetes, and systemic steroid use. Treatment is most often surgical, involving lens removal (patient becomes extremely farsighted); vision is corrected with glasses, contact lenses, or implanted plastic lens (intraocular lens).

The fovea centralis of the macula is the central focusing area and most sensitive portion of the retina. This region is thin because most of the other layers of the retina are absent. Here the photoreceptor layer consists only of cones, specialized for color vision and acute discrimination.

Accommodation of the Lens

The ciliary body contains smooth muscle arranged in a circular fashion like a sphincter (see Fig. 8.22 ). When relaxed, it pulls a set of zonular fibers attached to the elastic lens taut and flattens the lens for viewing objects at some distance from the eye. When focusing on near objects, the sphincter-like ciliary muscle (parasympathetically innervated by CN III) contracts and constricts closer to the lens, relaxing the zonular fibers and allowing the elastic lens to round up for accommodation (near vision).